1. Field of the Invention
The present invention is directed to a power amplifier of the type having at least one switched output stage with a power bridge circuit formed by switch elements connected to a floating intermediate circuit voltage, as well as with at least one digital pulse width modulator that generates pulse-width-modulated control signals for all of the switch elements of the power bridge circuit from digital input signals in order to generate an output stage voltage according to an output stage switch clock.
2. Description of the Prior Art
In power amplifiers of this type, high powers must be regulated with extreme accuracy. This is particularly the case for gradient amplifiers in a magnetic resonance tomography apparatus. The same is true, for example, in inductive heating devices in x-ray units or for the drive control of electric motors.
In a gradient amplifier, an alternating current on the order of magnitude of xc2x1300 V given a current on the order of magnitude of 300 A is generated with a power bridge circuit. The power amplifier must exhibit such high precision that the current flow for each of the three gradient coils can be set in the mA range. The turn-on phases of the individual switch elements in the power bridge circuit, which are essentially defined by the output stage switch clock or clocks, must therefore be capable of being essentially continuously varied as to their respective time duration. For this reason, pulse width modulators of gradient amplifiers have conventionally been implemented in purely analog fashion, in order to allow the switching times of the switch elements, which, for example, can be power transistors, to be controlled as finely as desired.
Particularly power amplifiers having a number of switched output stages require a high component outlay due to the corresponding number of analog pulse-width modulators and also require correspondingly complicated electrical connections since, in this case, a number of phase-shifted delta-shaped voltages are required. The required, high number of components in the known power amplifiers leads to a correspondingly large structural volume as well as to correspondingly high manufacturing costs.
German Patent 197 09 767 discloses a method for operating a power amplifier of the above-described type with a number of switched output stages wherein the pulse-width-modulated control signals are cyclically exchanged between the switched output units. As a result, a good distribution of energy that, for example, is fed back to a load, is achieved among all switched output stages without requiring special discharge or energy distribution means between the switched output stages.
A power amplifier of the type initially described is disclosed, for example, in U.S. Pat. No. 4,673,887. A digital pulse width modulator has, as the basic components for generating pulse-width-modulated control signals, a clock generator, a clock divider as well as at least one shift register. The clock divider generates an on/off pulse sequence from a clock signal of the clock generator, this on/off pulse sequence being supplied to the shift register. The shift register is fashioned such that a phase relation of the on/off pulse sequence can be shifted. The pulse-width-modulated control signals are ultimately formed from a logic operation of the phase-shifted on/off pulse sequence with the non-phase-shifted on/off pulse sequence.
The control of the digital pulse width modulator ensues via digital input signals that, for example, are supplied to the shift register. The digital input signals are formed from an analog input signal by an analog-to-digital converter that precedes the digital pulse width modulator.
The aforementioned power amplifier is inflexible in accommodating changing demands as to resolution of the amplifier output voltage. A more highly resolvable amplifier output voltage requires a digital pulse width modulator, particularly a shift register having a greater bit width, as well as an analog-to-digital converter having a greater bit width. In particular, the use of a shift register having a greater bit width involves complicated modification of the entire pulse width modulator and is correspondingly cost-intensive.
It is therefore an object of the present invention to provide a power amplifier of the type initially described wherein the aforementioned disadvantages of the prior art are alleviated.
This object is inventively achieved in a power amplifier according to the switch behavior of the digital pulse width modulator simulates the switch behavior of an analog pulse width modulator. The generated output stage voltages thus have a substantially analog curve, i.e. an analog curve or a nearly analog curve.
Inventively, a pre-modulator precedes the digital pulse width modulator, a prescribable number of input signals for the digital pulse width modulator being initially supplied thereto. Subsequently, the output signals formed in the pre-modulator can be supplied to the digital pulse width modulator as input signals.
By employing a pre-modulator, the resolution of the pulse-width-modulated control signals is greatly improved. The inventive power amplifier thus delivers exact output stage switch clocks, so that the current flux generated in the inductive load that is across at the outputs of the power amplifier exhibits a high precision. The inventive power amplifier is thus ideally employable for operating gradient coils in a magnetic resonance tomography apparatus.
The digital pulse width modulator utilized in the inventive power amplifier has a significantly smaller structural size compared to an analog pulse width modulator. The inventive power amplifier thus requires less installation space.
In an embodiment the resolution of the output signals of the digital pulse width modulator by push-pull dithering or, respectively, by isoclock dithering.
In a preferred embodiment of the invention, at least two switched output stages are series-connected at the output side so that the power amplifier has an output voltage that corresponds to the sum of these output stage voltages. In a further version, all switched output stages can be driven with switch signals that are offset relative to one another. As a result of this measure, the maximum output voltage as well as the effective switching frequency are multiplied by a factor that generally corresponds to the number of switched output stages.
By employing a number of switched output stages, the cost/power relationship is further improved, since, in addition to the economic, digital pulse width modulators, the individual switched output stages from which the power amplifier is constructed need not satisfy any particularly high demands and are thus more economic than a single, high-performance switched output stage. The advantages of inexpensive power transistors, which might otherwise be unsuited because they have a slow switching behavior, (for example, IGBTs, insulated gate bipolar transistors) can also be exploited. Moreover, due to the low switching frequency of the individual switched output stages, significantly lower losses occur.
In preferred embodiments, an odd numbered plurality of switched output stages is used. The phase angle of the output stage switch clock signals preferably is to 360xc2x0/k, wherein k is the number of switched output stages.
A uniform division of the overall load among the individual switched output stages preferably ensues. The switched output stages can, in particular, contribute to the output voltage of the power amplifier in equal parts and/or in a symmetrical fashion. For example, the switched output stages can be driven such that they supply voltage pulses of identical width.
Given an output voltage of 2000 V, a voltage boost of 400 V is obtained given, for example, five identical switched output stages, and thus a lower ripple is obtained at the output of the power amplifier despite a maximum output power of 2000 V.
In a preferred embodiment of the inventive power amplifier, two voltage pulses are generated in each switched output stage in each cycle of the output stage switch clock, each of these being separated by a free-running mode. The two voltage pulses can be produced by a diagonal operation of the power bridge circuit, and the two free-running modes can each correspond to a condition of the power bridge circuit wherein a load current can flow unimpeded through the power bridge circuit.